Since the beginnings of coastal or astronomic navigation, methods have been used to locate an object by comparing its apparent position relative to known positions of listed objects called landmarks.
Another known method is to proceed with location by manually or automatically comparing images of the object with reference data such as mapped images or data recorded on the ground. The results obtained are sensitive to the diachronism between these two sources (notably the differences of orientation between the images of the object and the mapped images, to the time-stamping and spectral band differences of the two sources). Improvement of the instrumental devices for measuring platform positions and attitudes in real time, for modeling the observation equations that describe the image exposure geometry, of the digital estimation techniques and of the computation means have made it possible to make systematic and automate certain basic processing operations to facilitate the interpretation and the superposition or merging of geographic data (stereo-restoration, aerotriangulation, rectification, etc.). These techniques have contributed to reducing the impact of the orientation errors of the snapshots on the geolocating of the image content.
These approaches have a number of drawbacks:                the gathering of objects or of field recordings on the sites represents a significant cost and remains constrained by aging with the evolution of the environment over time, whether natural or artificial,        the comparison with the object presents a few specific drawbacks (real time, robustness and image content constraints) with limitations due to poorly textured backgrounds such as, for example, in the case of a ship on the ocean, or on the other hand due to strongly textured backgrounds exhibiting quasi-periodicities,        the preparation to embed and process large and specific volumes of information,        the need for manual intervention, however minor, to correct any deviation is incompatible with a rapid preparation of the information.        
In the case where there is no a priori information concerning the object, such as the size or the volume, the techniques that use a specific knowledge of these characteristics are not eligible. Furthermore, the performance levels attained by these techniques for objects at a great distance are, moreover, highly inadequate when it comes to the desired quality objective. Metric class location performance levels are in fact targeted.
For the operations that require analysis of the image information onboard the platform and during the gathering process, another locating procedure, called direct geo-referencing technique, has been developed. It is based on an instrumental device, embedded onboard a platform, consisting of a device for acquiring images of the scene where the object is situated, means for positioning the platform in a terrestrial coordinate system, such as a GPS (Global Positioning System), and means for determining the attitude of the image, including:                an inertial navigation unit (INU) on the platform to determine its orientation in the terrestrial coordinate system,        means for measuring the alignment of the sensor on the platform to determine the orientation of the sensor in the coordinate system of the platform,        means for orienting the line of sight (LoS) relative to reference axes of the sensor, by means of mechanical elements (coders) or inertial elements (IMU).        
Once the image is acquired, two variants can be used to determine the geodesic position of the object:                the first uses the help of an altimetric model of the scene such as a digital terrestrial model assuming that the object is located on the Earth's surface; the position of the object is then obtained by intersecting, from the position of the sensor, the absolute direction defined by the image attitude with the terrestrial model (of the two solutions obtained, the correct position of the object corresponds to that of the shortest distance, the other is situated opposite the Earth's surface);        the second uses a distance measurement on the object by means of a range-finder embedded in the sensor and harmonized with the image.        
For the operations that allow a delay in the use of the image information onboard the platform, location techniques based on stereoscopic vision can be used to determine the location of the object discretely. These methods still suffer from the quality of the attitude measurements, from the limited accuracy of the distance measurement obtained by the stereoscopic vision, and from the difficulties in robustly and automatically associating the image characteristics of the object. The patent application FR 06 01125 can be cited on this subject.
The quality of the attitude measuring components, of affordable cost, such as the magnetic compasses remains limited to a performance level in the order of around ten mrad because of the poor knowledge of the terrestrial field, without even beginning to take into account the difficulties of integration in the structure or of the local disturbances. Despite the evolution of the inertial components, the trend indicates that it will be several more years before we have, through these techniques, a geolocation of metric class and notably in exposure conditions of interest corresponding to strongly oblique views (or at a great distance from the object) in which the elevation angle error becomes highly detrimental to the planimetric geolocation error.
In fact, the inertial unit is used to find the heading, the attitude and/or the speed and the position of the platform. Its quality is limited by the performance of the sensors of which it is composed (gyroscopes, gyrometers, accelerometers, etc.). The threshold reached by an inertial unit of aeronautical class is in the order of the mrad which is too high for the performance levels sought. These systems also require an initialization and a correction of the drifts over time. Finally, it should be noted that access to a metric class location would require inertial components of high performance whose price is beyond reach for aero-terrestrial equipment.
The mounting of the sensor on the platform introduces a systematic error which requires an angular bias calibration procedure. This calibration is ideally performed in real time and dynamically since it depends on the thermal, mechanical or geomagnetic conditions of use.
These various errors are aggregated in the optronics acquisition system. The order of magnitude of these errors will be illustrated by a quantified example. The inertial unit typically has a random error of 1 mrad; the alignment means typically have a static bias of 10 mrad; the LoS orientation means typically have a random error of 3 mrad. In the absence or after estimation of the bias, there remains an overall random error of approximately 3 mrad which is reflected, for example, in an error on the ground of 70 m for an object on the ground situated at 10 km acquired from an observation point at 20 kft. The aim is for a metric performance, and at greater distances.
It is also possible to cite the geolocation methods that use lateration techniques (tri or multilateration) which are based on measurements of determined distances for example from the attenuation of a signal or a time of flight (ToF) for the most powerful among them. The following techniques have the drawback of the need for cooperation with the object or/and an electromagnetic (EM) emission constraint on the part of the latter:                the conventional GPS uses distance measurements by measuring times of propagation (TOA, standing for “Time Of Arrival”) of radiofrequency signals, and cooperative receivers to locate a cooperating receiver; the position of the fixed or mobile object is determined by the intersection of spheres.        GSM (Global System for Mobile communication) is based on time measurement differences (TDOA, standing for “Time Difference Of Arrival”) for positioning a personal receiver of cell phone type within a collaborating network of antennas; the position of the fixed or mobile personal receiver is determined by the intersection of hyperbolas. The performance level that can be accessed is 150 meters in an urban area and 5 kilometers in a rural area.        ESM, standing for “Electronic Support Measure” is a passive listening sensor which makes it possible to measure the angular direction of arrival of EM radiations emitted by a fixed object, and which thus locates it with a technique usually based on time measurement differences (TDOA).        
More recently, cooperative techniques between airborne and terrestrial platforms have emerged. In these scenarios, either the terrestrial platforms produce a location of greater quality because they are at a shorter distance and are performed from fixed stations, or the airborne platform uses the positions communicated from the cooperating terrestrial platforms present within its image of interest as landmark point to perform the georeferencing of the image by a conventional photogrammetry technique.
The drawbacks of the techniques listed above mean that it is not possible to envisage geolocating a target or an object in the following conditions of implementation:                object inherently non-cooperative and not emitting EM radiation, automatically, therefore without manual intervention of the user during the process,        robustly with respect to the environmental context, that is to say, independently of the nature of the structures present (or absent) in the scene,        by systematically guaranteeing a solution regardless of the position of the object in space, notably in strongly oblique sight conditions,        with high accuracy and precision compatible with metric class requirements,        without requiring any calibration procedure,        without requiring attitude measuring means (inertial, magnetic, astrometric),        without a priori or hypothetical information concerning the knowledge of the object,        without embedding or having information concerning the environment of the object,        by dynamically estimating the quality on the position obtained,        by being able to adapt the performance level to achieve a metric setpoint level,        without systematically requiring the implementation of complex optimization processes,        by being able to operate in conditions of maximum discretion and at a great distance from the object,        independently with respect to communication means and cooperating infrastructures.        
Consequently, there is not currently any system or solution available that simultaneously meets all the abovementioned requirements.